Add type annotations to remove warnings and fix a bug.

lib/compiler/implementation/dart_backend/backend.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class ElementAst {
   final Node ast;
   final TreeElements treeElements;
 
   ElementAst(this.ast, this.treeElements);
 
@@ skipping 308 matching lines @@
       if (element.isMember()) {
         ClassElement enclosingClass = element.getEnclosingClass();
         assert(enclosingClass.isClass());
         assert(enclosingClass.isTopLevel());
         assert(shouldOutput(enclosingClass));
         addClass(enclosingClass);
         classMembers[enclosingClass].add(element);
         processElement(element, elementAst);
       } else {
         if (!element.isTopLevel()) {
-          compiler.cancel(reason: 'Cannot process $element', element: element);
+          compiler.cancel('Cannot process $element', element: element);
         }
         addTopLevel(element, elementAst);
       }
     });
 
     // Add synthesized constructors to classes with no resolved constructors,
     // but which originally had any constructor.  That should prevent
     // those classes from being instantiable with default constructor.
     Identifier synthesizedIdentifier =
         new Identifier(new StringToken(IDENTIFIER_INFO, '', -1));
@@ skipping 196 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().uri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Collection<Element> elements) =>
     sorted(elements, compareElements);